function bbp_get_forum_group_ids( $forum_id = 0 ) {

	// Assume no forums
	$group_ids = array();

	// Use current group if none is set
	if ( empty( $forum_id ) ) {
		$forum_id = bbp_get_forum_id();
	}

	// Get the forums
	if ( ! empty( $forum_id ) ) {
		$group_ids = get_post_meta( $forum_id, '_bbp_group_ids', true );
	}

	// Make sure result is an array of ints
	$group_ids = array_filter( wp_parse_id_list( $group_ids ) );

	// Filter & return
	return (array) apply_filters( 'bbp_get_forum_group_ids', $group_ids, $forum_id );
}